A projector apparatus using liquid crystal or a DMD (Digital Micromirror Device) condenses light emitted from a light source (discharge lamp), by using a reflection mirror and a lens system, and irradiates a small device which displays image information, therewith, whereby light reflected or transmitted from the small device is irradiated to a screen through an optical system such as a lens etc. The discharge lamp is required so as to be a point light source. When the small device is as small as 1 inch or less and the angle component of incident light flux is smaller, the use efficiency of light becomes high and the contrast of an image also becomes good.
There are a single plate type and a three-plate type system as a method for projecting color image information. Recently, the single plate system has been mainly adopted due to miniaturization of a projector apparatus. The single plate system is also called a color sequential system. The DMD is irradiated with incandescent light emitted from the light source through a color wheel, which is divided RGB (red, green, blue) areas, and selected light is reflected by the DMD so as to irradiate a screen with the light. The DMD has the structure in which millions of small mirrors are laid for every pixel, and projection of light is controlled by controlling the direction of each small mirror. A configuration example of such a light source apparatus of the projector apparatus is disclosed in Japanese Laid Open Patent No. 2005-196011.
Where an alternating current extra-high pressure mercury lamp is used for a color sequential type projector, polarity reversal of lamp current is in general synchronized with and spoke time of the color wheel (switching timing between color areas) thereby operating the projector. This is because, if the synchronization is not taken, an optical output changes at the time of the polarity reversal of the lamp current, so that flickering occurs on a projection screen.
Japanese Laid Open Patent 2003-51282 discloses that, in an alternating current extra-high pressure mercury lamp, a projection is formed at the tip of an electrode, in order to control unstable electric discharge. Japanese Utility Model Publication No. 2006-332015 discloses that specially, an extra-high pressure mercury lamp in which mercury whose amount is 0.20 mg/mm3 or more is enclosed, is operated at a lighting frequency in which a comparatively high frequency and a low frequency are combined with each other, since it is necessary to maintain and stabilize the projection of the electrode so as to have the appropriate shape, in order to stabilize an arc luminescent spot and to control the flicker phenomenon.